1. Field of the Invention
The present invention relates to methods and apparatuses for fabricating semiconductor devices on substrates, including semiconductor wafers. More particularly, the present invention relates to the formation of film layers on substrates by sputtering, wherein holes and/or trenches in the uppermost film layers must be uniformly, conformally filled.
2. Background of the Art
Sputtering is one well known method of depositing a film on a semiconductor substrate. A typical sputtering apparatus includes a target and a substrate support pedestal enclosed in a vacuum chamber. The target is typically affixed to the opposite wall from the substrate of the chamber, but is electrically isolated from the chamber walls. A voltage source maintains the target at a negative voltage with respect to the walls of the chamber, to convert a gas, maintained in the chamber at a low pressure, into a plasma. Ions from this plasma sputter the target. As target particles are sputtered from any point on the target, their trajectories theoretically have a cosine angular distribution; that is, the density of sputtered particles ejected from a point on the target along a trajectory having a given angle from perpendicular to the target is proportional to the cosine of such angle. Where the distance between the target and the substrate is less that the mean free path between collisions of atoms in the chamber, the target particles sputtered from the target generally travel in a straight line path and will tend to deposit on any surface which they contact.
One application of sputtering is to provide a continuous conformal metal film layer on the surfaces of holes and trenches extending through one or more pre-existing metal, dielectric or semiconducting film layers on the uppermost surface of the substrate. Each of the holes or trenches will typically include a base located substantially parallel to, and inwardly of, the uppermost film layer surface on the substrate, and a wall extending from the base to the exposed surface of the uppermost film layer.
One use of a conformal film layer is as a diffusion barrier at the base of the holes or trenches. For example, aluminum cannot be deposited directly on a silicon layer, because the aluminum will diffuse into the silicon and alter its properties. However, the low cost and high workability and conductivity of aluminum often dictate its use in semiconductor devices. Therefore, where aluminum is used to form a contact with a silicon layer, a barrier material such as titanium nitride is first deposited over the silicon.
To ensure that each of the contacts or vias formed with the metal layer have the same electrical properties, the thickness of the metal film layer deposited on the base of any of the holes or trenches should be the same thickness as the thickness of the metal film layer formed on the base of every other hole or trench, and the thickness of the metal film layer deposited on the wall of any of the holes or trenches should be the same thickness as the thickness of the metal film layer formed on the wall of every other hole or trench. Further, the film layer deposited on the wall of each of the holes should be symmetric, and the film layer should also have a relatively uniform thickness over the entire span of the wall or base of each hole or trench. However, the film layer formed on the base of the hole may be of a different thickness than the film layer formed on the wall of the hole.
The uniformity of the film layer deposited on the wall and base of each hole or trench is dependant on the distribution of the individual particles of target material reaching each of the holes or trenches. Particles travelling in paths which are substantially perpendicular to the substrate surface will pass through the open end of the hole or trench and deposit on the hole or trench base. Particles travelling at angles from perpendicular to the substrate surface will typically deposit on the hole wall and the intersection of the hole wall and base. However, particles that are travelling in paths at a substantial angle with respect to perpendicular from the substrate, or, in other words, are travelling in paths at low angles with respect to the substrate surface, may only reach the top most portion of the wall because the surface of the substrate at the hole opening shadows the base and lower portions of the wall from these particles.
Where a hole is deep with respect to the hole width, the target material which would commonly reach the lower reaches of the wall, or the intersection of the base and wall, is shadowed by the adjacent surface of the substrate. The blocked material deposits on the uppermost surface of the substrate adjacent to the hole opening, and this deposit can further block access of the target material to the lower reaches of the wall and the intersection of the base and the wall. The resulting film layer in the hole will be thickest at the center of the base and at the outermost areas of the wall but thin at the intersection of the base and the wall, leaving a less-filled region within the hole. Further, the deposit at the hole or trench opening will tend to restrict the hole opening, or it may actually form a bridge across the hole or trench opening to create an encapsulated void in the hole.
Optimally, the quantity of target material reaching each location on the hole base is equal so that a uniform thickness, conformal film layer will be formed over the base. Likewise, the quantity of material reaching each location on the wall is optimally equal, so that a symmetric, uniform thickness film layer is formed on the wall and no notches should be present where the wall and base layers meet. However, where the film layer is used as a diffusion barrier to prevent diffusion between a second film layer and the material at the base of the hole, the thickness at the base of the hole is the critical material thickness, and the wall coverage may be minimal.
The geometry of the holes and trenches, in combination with the distribution of the paths in which material sputtered from the target travel, prevents uniform conformal filling of the holes. As a first order approximation, the density of target particles travelling in a path at an angle to perpendicular to the target is proportional to the cosine of that angle. However, in reality the distribution of particles sputtered from each point on the target is not cosine angular but instead more complex, and is dependent on the type of target material being sputtered.
For example, where aluminum or aluminum alloy targets are sputtered, as an approximation, the distribution of target particle trajectories is proportional to the cosine of the angle formed between the target particle trajectory and any ray, or line, extending from the sputtered location at an angle of approximately 30.degree. to 40.degree. from perpendicular to the sputtering target surface. The resulting distribution of target particle trajectories is generally conical, wherein the greatest percentage of particles sputtered from the target travel away from the target in a conical distribution centered about a conical reference surface located at about 30.degree. to 40.degree. from the reference line perpendicular to the target. Thus, where aluminum is sputtered from a flat target, the majority of the target material travels in paths which are oblique, (i.e., substantially non-perpendicular) to the surface of the target.
Therefore, there exists a need in the art for a sputtering apparatus which provides a uniform film layer, while enabling void free, uniform, conformal covering of holes or trenches in the upper surface of the substrate.